1. Field of the Invention
The present invention relates to a new method and apparatus for monitoring and/or measuring cardiac output and SVR of a patient by analyzing a blood pressure signal which contains various parameters related to particular characteristics of the patient's vascular system.
2. Description of the Related Art
Cardiac output is the ultimate expression of cardiovascular performance. The term "cardiac output" indicates the quantity of the blood ejected each minute by either the right or left ventricle. Absolute cardiac output in an in vivo pulsating flow system is the product of the stroke volume and the heart rate frequency per minute. Stroke volume quantitatively defines the beat-to-beat volumetric performance of the heart as an intermittent flow generator or pulsating pump. The performance of the pump on a beat-to-beat basis and thus the magnitude of stroke volume is determined by three major independent variables: pre-load, contractility and after-load. Each major determinant of stroke volume has its own subsets of major and minor independent variables. The integrated modulation of all variables contributes to the stroke volume generated by the ventricles for each heartbeat. The parameters associated with cardiac output are useful in that they can be employed to evaluate the overall cardiac status of critically ill patients, patients with suspected cardiovascular and pulmonary diseases, patients undergoing surgery, and any situations that require blood pressure monitoring.
The radial stretch of the ascending aorta involved in the ejection of blood by the left ventricle initiates a pressure wave which propagates down the aorta and its various branches. The pressure wave travels with a finite velocity that is considerably faster than the actual forward movement of the blood itself, and is a wave which pulsates as it reaches the peripheral arteries. The velocity of transmission of the pressure wave varies inversely with the vascular capacitance of the arteries. Also, it is known that the velocity increases progressively as the pulse wave travels from the ascending aorta toward the peripheral regions. The arterial pressure signal contour becomes distorted as the wave is transmitted down the arterial system. There are three major changes which occur in the arterial pulse contour as the pressure wave moves forward. The first is that the high frequency components of the pulse, such as those corresponding to the dicrotic notch, are filtered and soon disappear. The second change is that the systolic portions of the pressure wave become narrowed and elevated. Third, a "hump" may become prominent on the diastolic portion of the pressure wave.
In elderly patients with less compliant arteries, the pulse wave may be transmitted virtually unchanged from the aorta to the periphery. The reason for this change is controversial. A common explanation is based on the concept that the pulse wave is reflected from branch points back toward the aortic arch to thereby set up pressure oscillations. In an elastic tube, a traveling pressure wave is reflected to some extent wherever there is discontinuity in the system. If the tube is completely blocked, reflection of the pulse wave energy is complete (and will be 180.degree. out-of-phase). If a pressure wave travels with increasing velocity toward the periphery and reflects back from regions where many branches occur over a short distance, the oncoming pressure wave is distorted, attaining a higher peak pressure and wider fluctuations following the peak, while at the root of the aorta, the initial upstroke of pressure is extremely rapid.
During the remainder of the systolic period (or "systole"), the pressure wave is rounded or "dome" shaped. The end of systole is clearly marked by a sharp dicrotic notch accompanying closure of semilunar valves. During the diastolic run-off, the pressure declines almost linearly. The pressure descends rapidly, and during the run-off period there is an additional wave called the dicrotic wave.
In addition to distortion of the waveform resulting from reflected waves, changes in the pulse waveform can be visualized in terms of its frequency contents. The transmission velocity of the high frequencies of the signal is faster than that of the low frequencies. Under these circumstances, the more rapidly traveling high frequency waves may produce increased peaking of the pressure pulse and corresponding deformation of the remainder of the pulse.
Various methods are known in the art for measuring cardiac output, a common approach involving a thermodilution technique in which a solution colder than body temperature is injected into the right atrium through a catheter and the resulting drop in blood temperature at the catheter tip indicates an amount of blood flowing around the tip. This method is invasive, however, and thus involves risks to the patient as it has the possibility of damaging the anatomical structures through which the catheter is threaded. Complications associated with pulmonary artery catheterization include pulmonary artery rupture, balloon rupture, sepsis, air embolism, etc.
Other conventional ways of determining cardiac output include thoracic bioimpedance and continuous wave Doppler ultrasonography. These two techniques are non-invasive, and are thus preferred for patients with high-risk vulnerability to the invasive procedures such as thermodilution estimation methods. In the bioimpedance method, changes in resistance to microcurrents injected into a patient are measured in order to calculate stroke volume, i.e., the amount of blood pumped in a single beat of the heart. The method therefore involves a pulse-by-pulse determination of cardiac output whereby four pairs of surface ECG electrodes are placed on the neck and chest of a patient. The outer pairs of electrodes inject a 70 KHz, 2.5 mA current into the thoracic tissue and the current is then sensed with the inner pairs of electrodes. The resistance to the injected current is dependent upon the fluid characteristics of the thoracic volume. Pulsating changes in thoracic resistance (bioimpedance) are then timed to the ventricle electrical depolarization and mechanical systole.
Continuous wave suprasternal Doppler ultrasound uses a Doppler transducer placed in the suprasternal notch directed toward the ascending aorta. The Doppler probe measures the aortic blood velocity. The integral of aortic systolic blood velocity multiplied by the cross sectional area of the aorta gives the stroke volume.
Other methods of cardiac output measurement are based upon the Fick principle. This method is simply an application of the law of conservation of mass, whereby according to this principle, the rate of uptake or release of substance to or from blood at the lung is equal to the blood flow past the lung and the content difference of the substance at each side of the lung. This method is most commonly used with oxygen as the analyzed substance. By means of in-dwelling catheters, arterial and venous blood samples were obtained and these samples were analyzed on a blood gas analyzer to obtain the oxygen saturation and the partial pressure of oxygen.
The Fick technique has the disadvantage that the measurements are complex and can often require an entire day of analysis before cardiac output can be ascertained. This makes the Fick technique undesirable in real-time applications where quick results are required in order to keep the patient in a stable condition.
There has been disclosed in the prior art, a technique for monitoring system vascular resistance (SVR) using derivative calculations of a blood pressure signal of a patient. In this method, the signal is differentiated and the points of greatest slope in the systolic portions are determined and then divided into the pressure which exists at these points. Such a calculation is proportional to the system vascular resistance, and multiplication by a resistance factor will yield the SVR value. However, the present inventors have discovered that this method may lead to inaccurate results in that the analysis of the blood pressure waveform does not take into account various factors which may lead to undesirable results. The inventors have found that the arterial waveform inherently contains various "artifacts" which cause errors in measurement of cardiac output. These "artifacts" are due to variable characteristics of the cardiovascular system such as different arterial capacitances of patients' arteries, reflected waves which cause aberrations in the signal, and also various damping characteristics of the waveform. There has thus been a need in the prior art for a technique which overcomes these drawbacks.